The present invention relates to power transmission systems and, more particularly, to apparatus for detecting and monitoring subsynchronous current in such power transmission systems.
The existence of asynchronous oscillations, i.e., oscillations at frequencies other than the generator's normal frequency, in power systems and series-compensated transmission lines has been known for years. Asynchronous oscillations occur as a result of the interaction or resonance of the system's capacitive and inductive components in response to transient conditions such as switching surges, fault initiation, and clearing. Usually, the oscillations are quickly damped by the resistive losses of the system and the natural or resonant frequencies do not precipitate other resonance reaction elsewhere in the system.
However, as the series capacitor compensation technique is increasingly employed for optimizing transmission capability of long high-voltage and extra-high-voltage lines, the expectancy that the natural frequencies of the asynchronous oscillations could and would fall below the 60 hertz (Hz.) synchronous frequency of the system becomes greatly increased. When this happens, mechanical resonance response in the generating equipment, via the stator windings, might be triggered.
Once an electrically initiated mechanical oscillation takes place, sustained or growing subsynchronous oscillations can be expected in a fashion similar to that of a positive feedback system. This effect could lead to eventual malfunction or even destruction of the generating equipment. Recently, there have been reported existences of generator damage and failure caused by subsynchronous oscillations. Thus, detection, monitoring and control of subsynchronous oscillations have become critical problems in the utility industry.
It should be noted that any apparatus designed to accommodate the problems associated with subsynchronous currents must exhibit rapid detection capability and response selectively so that false alarms due to short-lived transients are not tripped. Furthermore, such apparatus must be possessed of a wide operating range, able to detect 0.01 per unit (p.u.) or less subsynchronous current (SSC) in the presence of a 60 hertz current of many p.u. or a subsynchronous current component of many p.u.